1. Field of Invention
The present invention relates to a time adjustment device that corrects the time based on signals from a positioning information satellite such as a GPS satellite, to a timekeeping device that has the time adjustment device, and to a time adjustment method.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on a known orbit, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time) with extremely high precision.
A GPS receiver that receives signals from GPS satellites must receive the TOW (Time Of Week) signal contained in the signals from a GPS satellite in order to get the time information transmitted by the GPS satellite. The TOW signal is the GPS time, and more specifically is the number of seconds from the beginning of each week. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2001-59864.
The received signals are transmitted from GPS satellites orbiting at an altitude of approximately 20,000 km, and the received data may therefore contain errors. A parity check is therefore applied to detect if there is an error in the data.
A parity check involves segmenting the transmitted data stream into a specified number of blocks, appending the result (parity bit) of an operation on the bits of each block to the end of each block, and transmitting this parity bit with the data. The receiver determines there are no errors in the data if the appended parity bit matches the result of the same operation applied to the bits of the data block.
If a different data stream with the same parity is received, however, the parity check may determine that the data stream is correct, and the time may therefore be adjusted based on the wrong time information.